This application claims the priority of German Application No. 197 33 208.0, filed Aug. 1, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a circuit for an alternator (generator) of a motor vehicle and a method for controlling the circuit.
A circuit of this kind is already known from German Patent document DE 32 27 602 C2 (see FIG. 3 thereof) in which the alternator of a motor vehicle is divided into two half-windings. The outputs of each half-winding are each connected with the inputs of a rectifier. Each of the outputs of one half-winding is connected with an output of the other half-winding though a switchable element. The switchable element in each case is a thyristor triggered as a function of the engine rpm. When the engine rpm of the motor vehicle drops below a certain value, the thyristors are triggered to connect the two half-windings electrically in series. Above this rpm, the two half-windings are connected electrically in parallel.
A similar circuit is also known from German Patent document DE 23 52 227 A1 according to which, as a function of the engine rpm, a winding of one half-winding is switchable into a series connection with a winding of the other half-winding. Here again switching takes place as a function of engine rpm, with central control being performed by a relay so that either a complete 5 series connection or a complete parallel connection results. The two half-windings are wound in the same direction and have the same terminal connections.
This is intended to take into account the fact that the engine rpm of the motor vehicle, and hence the rpm of the alternator and consequently the power generated in the alternator, changes abruptly during operation. In order to provide sufficient electrical power even in urban stop-and-go traffic without imposing a load on the battery, the alternator must therefore be designed to generate sufficient power even when the engine is idling.
However, if the alternator is designed to have a lower cut-in rpm, power output is limited at higher rpm values. Conversely, if the alternator is designed for maximum power output, this requires a higher cut-in rpm. Hence, a compromise between these two criteria must be found. The conditions are shown for example in FIG. 2 of German Patent document DE 32 27 602 C2 in which the characteristic of the alternator power is plotted as a function of rpm. It is clear that the alternator has a so-called cut-in rpm above which it begins generating power. In order to generate sufficient electrical power during idle as well, it is necessary to lower this cut-in rpm point as far as possible. At higher rpm values, there is then a corresponding theoretical power surplus which is not useful, however. In a motor vehicle, the voltage at the terminals is not random but set to an on-board voltage of 14 V. The alternator then delivers a power of 6 kW at 6000 rpm for example, but only 2 kW can be obtained from it. Therefore, an unnecessarily large amount of iron must be carried around in the alternator, thus making the alternator over sized.
While the two hale-windings and/or the individual windings of the half-windings are each connected pairwise in series at low engine rpm values, the output voltage of the alternator is increased at low engine rpm values. The alternator can therefore be designed so that for the same cut-in rpm and power P.sub.el the extreme rpm range from approximately 2000 rpm to 18,000 rpm is limited to the range from 1000 rpm to 9000 rpm. This produces considerable improvements in the efficiency of up to 15%, especially in the upper rpm range. Further, the alternator noise is reduced.
Other solutions to this problem are based on a so-called "step-up" or star-delta switching. The step-up is connected at low rpm values for the engine and increases the rpm of the alternator by a factor of 2 once again. At this transmission ratio (V-belt 1:2 or 1:2.7+step-up 1:1.7=1:4.59), the alternator rpm is high enough even at idle to provide sufficient power. With star-delta switching, at low rpm values the alternator is star-connected since the alternator voltage is then larger by a factor of 13 than with a delta connection. The alternator therefore delivers the power even at lower rpm levels. At higher rpm levels it switches back to the delta connection so as not to have any adverse effect on the power.
The goal of the present invention is to provide a circuit for an alternator or generator for a motor vehicle as well as a method for controlling an alternator or generator so that the alternator or generator supplies sufficient power even at low rpm and the output power of the alternator or generator is not limited at higher rpm levels in order to meet the power requirement. The terms alternator and generator can be used interchangeability herein. According to the invention, this goal is achieved firstly by a circuit in which, in contrast to the abovementioned prior art, each output of one half-winding can be connected across the gaps of two switchable elements with two outputs of the other half-winding in such fashion that when the gaps are bridged, the two half-windings are connected electrically in series. Therefore a star, delta, or zigzag winding can be used for the two half-windings.
It turns out that -he losses in the switchable elements can be minimized during operation at low rpm values. This can be demonstrated, for example, if the switchable elements are formed by thyristors as described in the circuit according to the invention. Losses are also minimized if the switchable elements are designed as MOS components.
In another preferred embodiment according to the invention, at least one output of one half-winding is connectable across the gap or gaps of at least one switchable element with a number of outputs of the other half-winding that corresponds to the number of switchable elements in such fashion that when the gaps are bridged, individual windings of the two half-windings are connected electrically in series.
Therefore, certain windings remain connected in parallel in all cases. In this embodiment, parts can be saved. At low rpm values, this circuit offers only a slighter power by comparison with the circuit according to the first embodiment, but can be sufficient under certain conditions that depend on the loads to be supplied with power. This circuit also halves the cut-in rpm.
In another preferred embodiment according to the invention, at least one output of one half-winding is connected by through at least one diode with a number of outputs of the other half-winding that is equal to the number of diodes in such fashion that individual windings of the two half-windings are connected permanently electrically in series.
In contrast to the prior art, in this case the individual windings are permanently connected electrically in series without the series circuit being switchable to a parallel circuit. In this circuit, the switching is considerably simplified to the point where no triggering of switchable elements is required.
This circuit produces a characteristic in which the power provided by the alternator at higher rpm values is limited relative to the circuit according to the first embodiment. Depending on the electrical loads (consumers) to be supplied, however, this reduced power can be sufficient. When the power that is made available on the basis of the characteristic curve is sufficient even at lower rpm values, a circuit can be produced in which no control over the switchable elements is required. The switching expense (cost) can then be completely simplified (reduced overall), and in this case the cut-in rpm of the alternator is also cut in half.
In a preferred advantageous circuit according to the invention, at least one additional output of one half-winding is connectable across the gap or gaps of at least one switchable element with a number of outputs on the other half-winding that corresponds to the number of switchable elements in such fashion that when the gaps are bridged, individual windings of the two half-windings are connected electrically in series.
Advantageously this results in a combination of windings of the two half-windings that are permanently connected (via the diodes) in series with other windings that are connected in series by means of the switchable elements when the switchable elements are triggered, and are otherwise connected in parallel.
According to the invention, a method for controlling the circuit for the alternator triggers the switchable elements simultaneously when the engine rpm of the vehicle drops below a certain value.
As a result, a switch is made from the characteristic curve of the parallel circuit to the characteristic curve of the series circuit. The alternator can then be operated at any rpm in accordance with these two switch states to supply a power that comes quite close to the theoretical alternator power that can be achieved. This theoretically achievable alternator power can be described as follows: EQU P.sub.theo =C.sub.Esson *D.sup.2 *li*n.sub.sy.
The individual values are the Esson coefficient C.sub.Esson, the bore diameter D, the ideal laminated core length li, and the synchronous rpm n.sub.sy.
In a preferred method according to the invention, the switchable elements are triggered sequentially and/or only individual switchable elements are triggered.
As a result, a kink in the power characteristic curve during the switching process can be advantageously avoided. Since the thyristors are triggered sequentially, there is a gentler transition. Therefore, there are no oscillatory excitations of the V-belt that could take the form of mechanical stress.
In a further preferred method according to the invention, the switchable elements are triggered when the engine rpm of the vehicle drops below a certain value.
Advantageously, at higher rpm values the two half-windings are operated in parallel, while a transition to a series connection is gradually performed during the transition to lower rpm values.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.